1. Field of the Invention
The present invention relates to a hot-dip galvanized steel sheet to be used as a corrosion preventive steel sheet for automobiles. More particularly, the present invention relates to a hot-dip galvanized steel sheet which has a good surface appearance free of bare spots even through its basis material contains Si and Mn which are said to adversely affect platability, said good surface appearance being produced by adequately controlling (in if terms of morphology) the Sixe2x80x94Mn concentrated phase occurring in the vicinity of the interface between the basis material and the zinc layer. Incidentally, the term xe2x80x9chot-dip galvanized steel sheetxe2x80x9d used herein embraces not only ordinary ones carrying the zinc layer as it is formed by dipping in the plating bath but also so-called hot-dip galvannealed steel sheets which have undergone heat treatment for alloying after galvanizing (to form the zinc layer on the basis material).
2. Description of the Related Art
There is an increasing demand for improvement in fuel consumption rate as a part of the policy for global warming due to CO2 discharge. To this end, a new target for improved fuel consumption has been set up and a new tax system has been introduced in favor of cars with improved fuel economy. One effective way of improving fuel consumption rate is by reduction of body weight. Achieving this object requires the raw material to have higher tensile strength than before. This is also the case with hot-dip galvanized sheet sheets. For hot-dip galvanized steel sheets to have both high tensile strength and good formability, the basis material (steel sheet) should be incorporated with such elements as C, Si, Mn, and Cr.
Unfortunately, such additional elements (particularly Si and Mn which are easily oxidizable elements) preferentially oxidize and concentrate on the surface of steel sheet during annealing in a reducing atmosphere, thereby greatly aggravating wettability and giving rise to bare spots detrimental to the external appearance. The reason for this is that hot-dip galvanizing is preceded immediately by annealing in a reducing atmosphere for reduction of Fe oxides on the surface (which is necessary for the steel sheet to exhibit good platability) and this annealing yields oxides of Si and Mn which are poor in compatibility with the galvanized zinc layer.
Consequently, it is essential that the formation of Si and Mn oxides should be minimized at the time of production of hot-dip galvanized high-tensile steel sheets. Among many other means to achieve this objective is an additional step preceding the ordinary annealing (for reduction) and hot-dip galvanizing, as disclosed in Japanese Patent Laid-open No. 34210/1995. This additional step consists of heating the steel sheet up to 400-650xc2x0 C. so that Fe is oxidized in the preheating zone of the annealing furnace whose atmosphere has an oxygen concentration of 0.1-100%.
Hot-dip galvanizing in the above-mentioned way depends on Si content in steel sheets for its effect and hence it is not necessarily suitable for steel sheets with a high Si content. It gives a zinc layer which is complete (free of bare spots) immediately after hot-dipping but peels off due to insufficient adhesion in succeeding fabricating steps, as demonstrated in Examples given later. In other words, Si and Mn cannot be added sufficiently because of restrictions imposed by platability although they are essential to improving the formability of steel sheets. Therefore, incorporation with Si and Mn is not a practical solution to the problem.
Another way of avoiding bare spots is by depositing Fe or Ni on the surface of steel sheet by preliminary electro-plating prior to annealing for reduction and hot-dip galvanizing. Electroplating unfavorably needs additional equipment and steps, leading to an increased production cost.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide a hot-dip galvanized steel sheet which has high tensile strength, good formability, and good surface appearance (free from bare spots) even though the basis steel sheet contains Si and Mn in a comparatively large amount and hence is prone to suffering bare spots.
The first aspect of the present invention resides in a hot-dip galvanized steel sheet composed of a basis steel sheet containing Si in an amount of 0.05-2.5 mass % and Mn in an amount of 0.2-3 mass % and a hot-dip galvanized zinc layer formed on the surface thereof, wherein said hot-dip galvanized zinc layer is formed in such a way that there is an Sixe2x80x94Mn enriched phase which is found, by observation under a scanning electron microscope or a transmission electron microscope, in the vicinity of the interface in a region no shorter than 50 xcexcm in the cross section perpendicular to the interface between the basis steel sheet and the hot-dip galvanized zinc layer, said Sixe2x80x94Mn enriched phase containing more than twice as much Si and/or Mn as the basis steel sheet and extending over a length no more than 80% of the length of the interface observed.
The second aspect of the present invention resides in the hot-dip galvanized steel sheet as defined in the first aspect of the present invention, wherein the Sixe2x80x94Mn enriched phase containing no less than twice as much Si and/or Mn as the basis steel sheet is found, by observation under a transmission electron microscope, in the boundary between grains or the inside of grains of the basis steel sheet within 1 xcexcm in the depthwise direction from the interface.
The third aspect of the present invention resides in the hot-dip galvanized steel sheet as defined in the second aspect of the present invention, wherein the Sixe2x80x94Mn enriched phase existing in the boundary between grains or the inside of grains of the basis steel sheet has a size no smaller than 5 nmxc3x975 nm.
The fourth aspect of the present invention resides in the hot-dip galvanized steel sheet as defined in the second aspect of the present invention, wherein the Sixe2x80x94Mn enriched phase existing in the grain boundary of the basis steel sheet has a length no less than 10% of the overall length of the grain boundary of the basis steel sheet in the field of vision of observation.
The fifth aspect of the present invention resides in the hot-dip galvanized steel sheet as defined in the first aspect of the present invention, which contains a compound no smaller than 5 nm in outside diameter, which is composed of atoms having an atomic number smaller than the average atomic number of atoms constituting the steel, in the boundary between grains or the inside of grains of the basis steel sheet within a range of 1 xcexcm in the depthwise direction from the interface, said compound being observed under a transmission electron microscope.
The sixth aspect of the present invention resides in a hot-dip galvanized steel sheet composed of a basis steel sheet containing Si in an amount of 0.05-2.5 mass % and Mn in an amount of 0.2-3 mass % and a hot-dip galvanized zinc layer formed on the surface thereof, wherein the basis steel sheet in the vicinity of the interface between the basis steel sheet and the hot-dip galvanized zinc layer contains Si or Mn in the form of solid solution such that its amount is less than 0.7 times the amount of Si or Mn in the composition of the basis steel sheet.